In the field of organic semiconductors, a major effort exists to improve the charge carrier mobility of the organic semiconductor material to levels approaching amorphous silicon (0.1 cm2/Vs) [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. This would enable organic field effect transistors (OFET) to operate at frequencies that make them attractive for a host of applications including radio frequency identification (RFID) tags and thin film transistors (TFT) in flat panel displays.
One organic semiconducting polymer candidate is regioregular polyalkyl thiophene (PAT) [see Z. Bao et al., Appl. Pys. Lett. 1997, 78, 2184]. This polymer can be fabricated via solution deposition, to form thin films with a highly ordered morphology, which facilitates charge transfer via a hopping mechanism.
Very promising mobilities of up to 0.1 cm2/Vs have been demonstrated for this polymer. See H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig & D. M. de Leeuw “Two-dimensional charge transport in self-organized, high-mobility conjugated polymers” Nature 401 685 (1999).
In order to increase this value further, and maximise the carrier mobility, one strategy is to increase the number of charge carriers, by doping the polymer. As described by A. R. Brown, D. M. De Leeuw, E. E. Havinga, and A. Pomp in “A Universal Relation between conductivity and field effect mobility in doped amorphous organic semiconductors”, Synth. Met. 68, 65–70 (1994), increased dopant concentration increases the density of states, and hence increases the probability of hopping transport. Thus, equally, an increase in charge mobility is expected with increase in dopant concentration.
The above reference also demonstrates that the bulk conductivity is also proportional to the number of mobile charge carriers per unit volume. A practical consequence of this however, is that the ON/OFF ratio of the OFET drops with increasing doping concentration, ultimately to a value of unity.
To circumvent this problem, new transistor structures and modes have been proposed [see Lloyd, G. C. R.; Sedghi, N.; Raja, M.; Di Lucrezia, R.; Higgins, S.; Eccleston, W. “Increasing the carrier mobility in P3HT by doping for use in schottky barrier TFTs”, Materials Research Society Symposium Proceedings (2002), 708 (Organic Optoelectronic Materials, Processing and Devices), 429–434], which utilise Schottky barriers between the electrodes and the semiconductor. By employing this design, the polymer can be fully doped, which can also lead to higher mobilities, without suffering the problem of low ON/OFF ratios.
A simple way to dope the semiconductor is to add an electron poor organic dopant like for example dichlorodicyano-benzoquinone (DDQ) (1) to the polymer formulation before fabrication, which leads to its incorporation in the polymer matrix. DDQ is known in prior art as an oxidative dopant small molecule containing effective electron withdrawing functionality such as quinoline and cyano groups [see C. P. Jarrett, R. H. Friend, A. R. Brown, D. M. DeLeeuw, in “Field effect measurements in doped conjugated polymer films: Assessment of charge carrier mobilities”, J. Appl. Phys, 77(12) 6289 (1995).].

A major problem with these small molecule dopants, is that they can be mobile, diffusing through the polymer film or subliming from the surface. Both these effects have detrimental consequences for the performance of a working OFET device, potentially leading to time dependant mobility, and high off currents.
It is an aim of the present invention to provide new methods of doping organic semiconductor materials to induce or increase their charge carrier mobility and electrical conductivity. Another aim of the invention is to provide new materials that are suitable as doping agents.
Another aim of the invention is to provide advantageous uses for the new doping materials, in particular for semiconductor, electrical conductor or photoconductor components or materials, and in optical, electrooptical or electronic devices.
Other aims of the invention are immediately evident to those skilled in the art from the following description.
The inventors have found that these aims can be achieved by using polymeric dopants, in particular polymers comprising an electron accepting group in the side chain, as doping agent in an organic semiconductor or charge transport material. The inventors have further found that polymers comprising a side chain based on quinone are especially useful as polymer dopants.